1. Field of the Invention
The present invention relates to an electrical control circuit, and more particularly to a control circuit for selectively manipulating the derivative function of a controller.
2. Description of the Prior Art
In the art relating to process control circuitry, one preferred form of such control circuit is known as a three-mode controller. That term refers to the three modes of characterization of an error signal which has been found desirable to accomplish a desired control function. In such circuits, a signal representative of a process variable is compared with a reference or set-point signal to produce an error signal, a signal representative of the difference between the measured value of the process variable and the desired value of the process variable. That error signal is then subjected to the three modes of characterization to produce a control signal. The first of these three modes is known as proportional control and is effected through controlling the amplification factor of the controller. The second mode of characterization is known as an integral function or "reset". This characterization produces a rising output signal so long as the measured variable deviates from the set-point value. The third mode of characterization produces an output signal which is a function of the rate of change of the error signal. In the three mode controller, the output signal is a function of the combined influence of all three of these characterization modes. Thus, a step change in the error signal would produce an ultimate output signal which has an initial high spike which diminishes to the level of the proportional signal then gradually rises at a rate determined by the circuit parameters so long as the error signal exists. This is a desirable characteristic when the step change in the error signal is occasioned by the sudden change in the process variable signal. A similar step change in the error signal would occur if the set-point signal were changed. Such a characterized output signal would not be desirable when the step change in the error signal is occasioned by a change in the set-point signal. In the known circuits heretofore provided, such as that shown in Gormley et al, U.S. Pat. No. 3,530,389, the circuit is incapable of distinguishing between a step change in the error signal caused by the process variable change or a step change in the error signal caused by a set point change. The portion of the characterized signal which is undesirable with respect to step changes occasioned by the change is set-point is the initial spike or derivative mode function.